When cutting sheets from a continuously moving web, it is important to match the tear-off speed, or removal or separating speed of the cut sheets to the subsequent handling apparatus. Customarily, the webs are fed at the linear speed which is less than the removal speed of the subsequent removal apparatus. This requires accelerating the sheets to the new, removal apparatus speed. It is also important to be able easily adjust the apparatus to handle sheets of different length being cut from the moving web.
In the book "Der Tiefdruck" published by Polygraph, 1952, page 206, a folding apparatus is shown in FIG. 187 which is provided to fold sheets which are cut from a continuous web. A continuously moving web is guided over a folding former, or folding triangle, and then moved through tension rollers to a pair of cutter cylinders, for cutting the folded web into folded sheets. The length of the folded sheets is determined by a forme cylinder positioned in advance-with respect to the direction of movement of the web-of the folding former. It is customary to operate folding apparatus of this type at a constant speed. The tension rollers which are placed in advance of the cutter cylinders may have circumferential speeds which are matched to the speed of the web. The speed of the web is always less than the linear speed with which the folding apparatus, and the cylinders and rollers thereof will operate. Consequently, the sheets which are cut from the web have to be accelerated. Since apparatus downstream of the cutters may work with grippers, sheets which are shorter than others have to be accelerated more. As the length of the sheets decreases, the acceleration has to be increased. The literature referred to shows a folding apparatus in which a removal or tear-off roller pair is used which is located downstream of the cutter apparatus within conveyer tapes or belts in order to guide the sheets. The sheets are accelerated at the instant of cutting them or separating them from the endless web to a speed corresponding to that of the subsequent removal apparatus, which may include the folder, by gripping or seizing the leading edge of the sheet to be cut by cams located on the tear-off rollers. The circumferential position of the cam discs on the tear-off rollers is moveable or changeable in order to be able to match the position of the cut sheet to the length thereof.
It has also been proposed-see page 100 of the above referred to text book-to utilize pivotable strips located on the tear-off of separating rollers which, when they meet the sheet, groups or grips the sheet. In order to accommodate sheets of different lengths, or to match apparatus of this type to different rotor reprinting machines of folding apparatus, it is necessary to so design the accelerating system that these pivoting strips can be circumferentially shifted about the circumference of the tear-off or separating rollers, since sheets of different lengths must be gripped at different instances of time with their leading edge. This, only, insures that the sheet to be cut is properly tensioned throughout its length at the time of the cutting, so that it can be accelerated without buckling or deformation.
It is not possible to change the cam location, or the location of the pivot strips in the known systems during operation; consequently, any adjustment or resetting is possible only when the machine is stopped. Cams which are to grip sheets may be subject to differential wear, and if the cams are located, axially staggered on the separating rollers, differently worn cams, or slightly skewed gripper-type pivoting strips will not uniformly grasp the sheet, resulting in skewed, or inclined pulling thereof. Use of cams has the additional disadvantage that the loading placed on the paper by the cams is very high due to the small surface area thereof, so that the specific surface loading may be so great that the substrate-if paper-may tear or be damaged; this, then, also leads to smear of printing subject matter due to the still-wet printing ink. It is thus necessary to an adjust the gripping cams or strips that they will engage only edge or marginal portions of the sheet, where no printed subject matter is contained. This, again, requires readjustment and resetting of the apparatus for specific jobs in accordance with the particular printed arrangement.
Use of separating tear-off rollers results in high noise levels during operation, since the engagement of the cams, and the pivoting or tipping of the pivot strips is noisy, often resulting in noise similar to hammering or impacts. The noise level of operation can be reduced by making the cams, or the pivot strips of high strength foam material. Unfortunately, however, the strength of such materials is insufficient for long time operation, and such foam structures, particularly of foam plastic materials require frequent replacement.